1. Field of the Invention
The present invention relates to a bearing rigidity evaluation apparatus for evaluating bearing rigidity of a bearing such as a double row bearing, a duplex bearing, or the like, to which a preload is applied.
2. Description of the Related Art
Heretofore, a double row bearing or a duplex bearing to which a preload is applied needs to have high rigidity in terms of performance of a machine equipped with the bearing. However, if the amount of the preload increases so that rigidity becomes too high, the excessive preload causes lowering of bearing performance (such as increase of frictional moment, abnormal heating, fatigue life, and so on). Accordingly, the value of bearing rigidity needs to be controlled in a predetermined range while being related to the amount of the preload.
The following apparatus is known as an apparatus for measuring the value of rigidity of a bearing. Japanese Patent Unexamined Publication No. Hei.5-10835 discloses a method in which a vibration signal generated from a rotating bearing is subjected to frequency analysis to thereby obtain both a contact angle of a rolling body and a resonance frequency of the bearing so as to obtain both the bearing rigidity and the amount of preload on the basis of the contact angle and the resonance frequency. Further, Japanese Patent Examined Publication No. Hei.2-61700 discloses a method in which vibrations given to a bearing box or a shaft by a vibrating machine is detected by a velocity or acceleration sensor and amplified by an amplifier, and an output signal of the amplifier is analyzed by a frequency analyzer to detect a resonance frequency of a bearing. The relation between the detected resonance frequency and the amount of preload is obtained in advance by calculation such as a finite-element method, or the like, so that the amount of preload can be detected on the basis of the resonance frequency.
Both the methods described in Japanese Patent Examined Publication No. Hei.2-61700 and Japanese Patent Unexamined Publication No.5-10835, however, utilize resonance. Accordingly, in the case where the value of rigidity of a duplex bearing is to be measured, when the rigidity of the bearing is relatively low, there is no problem. On the other hand, the rigidity of the bearing is high, a vibration mode due to the bearing as a structural body and a vibration mode due to a bearing spring are coupled with each other, so that it is difficult to detect rigidity caused by the bearing spring.
Further, even in the case of a bearing as a single body, the same problem as described above arises in accordance with local vibration modes of a low-rigidity portion-including bearing such as a flanged bearing and elastic vibration modes of inner and outer races. Accordingly, in a bearing requiring high rigidity, bearing rigidity is hardly detected on the basis of the resonance frequency by use of a vibration model having inner and outer races as mass points as described in Japanese Patent Unexamined Publication No. Hei.5-10835. Further, when external vibrations in a measurement frequency band are detected, S/N ratio with respect to the external vibrations also becomes a subject of discussion.